The fundamental measurements in gait analysis consist of joint motion and net joint kinetic estimates. The fact that such measurements are referenced to joints implies knowledge of the positions of the skeleton. Data on skeletal position are typically obtained by measuring the locations of targets attached to the surface of the body. Errors in these data are introduced by the movement of the targets relative to the skeleton, due to the combined effect of the target attachment technique and displacements of the skin surface relative to the bone. The magnitude and pattern of these "surface movement errors" in standard gait data remain unknown, due to difficulties in measuring the true skeletal motion. Available data suggest that surface movement errors may be of considerable magnitude. The purpose of this project is to investigate the magnitudes and patterns of the surface movement errors during walking. A principle objective necessary to achieve this goal was to develop and demonstrate a minimally invasive technique for making the measurements. The movement of surface mounted targets (SMT) on a shell at the mid-shank, and of bone mounted targets attached to the distal shank using a Percutaneous Skeletal Tracker (PST), were simultaneously measured during free-speed walking of three adult subjects having different body types. Surface movement errors in shank kinematic estimates were determined by expressing the segmental motion derived from the SMT relative to the PST-based segment coordinate system (SCS) located at the segment center of gravity. The greatest errors were along and around the shank longitudinal axis, with peak magnitudes of 10 mm of translation and 8 degrees of rotation in one subject. Estimates of knee joint center locations differed by less than 11 mm in each SCS direction. Differences in estimates of net knee joint forces and moments were most prominent during stance phase, with magnitudes up to 39 N in the shank mediolateral direction and 9 N.m about the mediolateral axis. The differences in kinetics were primarily related to the effect of segment position and orientation on the expression of joint forces and on the magnitude and expression of joint moments.